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Quiz: Centrifugal, Axial and Mixed Flow Fans

Test your understanding of fan families, housings, vanes, rooftop and lab exhaust fans, and duty classes with these review questions.

1. What defines a fan family?

  1. The airflow path through the impeller, relative to the shaft
  2. The shape of the housing that surrounds the impeller
  3. The duty class, commercial or industrial, in the catalog
  4. The drive type, direct drive or belt driven
Show Answer

The correct answer is A. A fan family is a group of fans that share the same airflow path through the impeller: centrifugal (in along the shaft, out at a right angle), axial (in and out along the shaft, with swirl) or mixed flow (out on a diagonal). The housing does not decide the family, because a tubular housing can hold axial, centrifugal or mixed flow impellers. Duty class and drive type are separate questions.

Concept Tested: Fan Family

See: Fan Families

2. Which description fits a mixed flow fan?

  1. Air leaves the impeller at a right angle to the shaft and is collected by a scroll housing
  2. Air enters and leaves along the shaft, and the tube has no vanes to remove the swirl
  3. Air leaves the impeller on a diagonal, and straightening vanes in a tubular housing return it to the duct direction
  4. One axial impeller and one centrifugal impeller are mounted together on the same shaft
Show Answer

The correct answer is C. A mixed flow fan uses a hybrid axial-centrifugal impeller, so air leaves it on a diagonal, partly along the shaft and partly outward. FanPedia groups mixed flow fans with tubular inline centrifugal fans: both have tubular housings, move air straight through, and use straightening vanes. The blades taper from the front plate to the hub end. Option A describes a housed centrifugal fan, and option B a tubeaxial fan.

Concept Tested: Mixed Flow Fan

See: Mixed Flow Fans

3. A drawing shows a round tube in a straight duct run, and the schedule labels it "tubular inline centrifugal." Which family does the fan belong to?

  1. Axial, because air enters and leaves the tube along the shaft
  2. Centrifugal, because the impeller throws air outward and vanes turn it back along the tube
  3. Mixed flow, because every tubular fan with stationary vanes is a mixed flow fan
  4. Vaneaxial, because the tube carries stationary vanes downstream of the impeller
Show Answer

The correct answer is B. The impeller decides the family, not the housing. In a tubular inline centrifugal fan the impeller throws air outward against the tube wall, and straightening vanes turn it back to the duct direction. From outside it looks like a vaneaxial, which is the trap. When a drawing is ambiguous, the schedule's fan type, the model designation or a look through the impeller access door settles it.

Concept Tested: Inline Centrifugal Fan

See: Inline Centrifugal Fans

4. What makes a vaneaxial fan different from a tubeaxial fan?

  1. A centrifugal impeller in place of the axial impeller
  2. A scroll housing in place of the plain tube
  3. Inlet vanes that pre-spin the entering air for capacity control
  4. A vane section of guide vanes that straightens the swirl leaving the impeller
Show Answer

The correct answer is D. Both are axial fans in tubular housings. The vaneaxial adds guide vanes downstream of the impeller, which turn the swirling air into straight flow and recover part of its energy as static pressure. That is why the catalog positions vaneaxials for moderate to high pressures. Option C describes inlet vanes, a flow-control accessory that customers sometimes call "inlet guide vanes"; they are a different part.

Concept Tested: Guide Vanes

See: Guide Vanes

5. Fume hoods exhaust 4,000 CFM of contaminated air. A bypass air plenum adds 2,000 CFM of outside air ahead of the fan, and the nozzle entrains another 4,000 CFM into the plume. What airflow should the fan be selected for?

  1. 4,000 CFM
  2. 6,000 CFM
  3. 8,000 CFM
  4. 10,000 CFM
Show Answer

The correct answer is B. The fan is selected for the fan flow, the air that actually passes through the impeller: contaminated air plus bypass flow, 4,000 + 2,000 = 6,000 CFM. Entrained flow joins the plume after the fan, so the total flow of 10,000 CFM (option D) is what leaves the nozzle, not what the fan moves. Option A ignores the bypass air, and option C wrongly adds the entrained air to the hood exhaust.

Concept Tested: Laboratory Fume Exhaust Fan

See: Laboratory Fume Exhaust Fans

6. How do the plenum type and plug type of unhoused centrifugal fan differ?

  1. A plenum type operates inside an air plenum such as an air handler cabinet; a plug type mounts through the plenum wall with its drive outside
  2. A plenum type has a scroll housing inside the plenum; a plug type has no housing at all
  3. A plenum type uses an axial impeller; a plug type uses a centrifugal impeller
  4. A plug type discharges through a single outlet flange; a plenum type discharges all around the impeller
Show Answer

The correct answer is A. Both are centrifugal impellers with inlet funnels and no scroll, so air leaves all around the impeller into the plenum. The plenum type is designed for air handling applications inside a plenum, such as an air handling unit cabinet. The plug type is "plugged" into the plenum wall: its mounting panel attaches outside the wall with the impeller inside, which suits high-temperature ovens where the motor and bearings must stay out of the hot zone.

Concept Tested: Unhoused Centrifugal Fan

See: Unhoused Centrifugal Fans

7. A schedule lists an exhaust fan as "PRV, hooded, belt driven" with no airflow or static pressure. What should a representative conclude before opening a catalog page?

  1. It is a centrifugal roof exhauster, because every power roof ventilator is centrifugal
  2. It is an axial roof ventilator, because every hooded roof unit uses an axial impeller
  3. The family is not yet known, because hooded PRVs exist in both families, so ask about ductwork and static pressure
  4. It is a gravity relief ventilator, because a hood shows that the unit has no fan
Show Answer

The correct answer is C. A power roof ventilator is a motor-driven fan unit for a roof opening, and it can be centrifugal or axial; both families have hooded, belt-driven versions (BCLH and THB in the current catalog). "PRV" tells you where the fan sits, not how air moves through it. A hooded axial unit suits high-volume, low-pressure exhaust, while a centrifugal exhauster handles more duct resistance, so the representative asks whether the unit is ducted and what static pressure it must overcome.

Concept Tested: Power Roof Ventilator

See: Power Roof Ventilators

8. A school needs restroom exhaust, and a foundry needs a fan to pull hot, dusty air to a dust collector. Both inquiries may end in a centrifugal fan. Which statement best explains how duty class relates to fan family?

  1. Duty class follows family: centrifugal fans are industrial duty and axial fans are commercial duty
  2. Duty class is set by fan size: fans above a certain diameter are industrial duty
  3. Duty class is set by drive: commercial duty fans are direct drive and industrial duty fans are belt driven
  4. Duty class comes from the airstream, setting and service, while family comes from the airflow path
Show Answer

The correct answer is D. Duty class and family answer separate questions. The family comes from the airflow path through the impeller. Duty class comes from the airstream, the setting and how hard the fan must work: clean, room-temperature air in a building where quiet matters is commercial duty, while a hot, particulate-laden process airstream is industrial duty, with construction options to match. Many families, including centrifugal and axial, appear in both of Twin City Fan's product lists.

Concept Tested: Industrial Duty Fan

See: Commercial and Industrial Duty

9. A technician photographs an unlabeled housed centrifugal fan from the drive side. Starting at the cutoff, the gap between the impeller and the scroll widens as the scroll runs counterclockwise toward the outlet. Which way does the impeller turn, viewed from the drive side?

  1. Counterclockwise, the direction in which the scroll passage widens
  2. Clockwise, because the air travels opposite to the impeller's rotation
  3. It cannot be told from the scroll; only the nameplate shows rotation
  4. Clockwise, because housed centrifugal fans always turn clockwise from the drive side
Show Answer

The correct answer is A. The impeller carries air around the scroll in its direction of rotation, and the passage grows in that same direction because more air joins the flow as each blade passes. Starting at the cutoff and following the scroll the way the gap widens traces the direction of rotation, so a counterclockwise path means counterclockwise rotation as viewed from the drive side. The scroll, not a fixed rule, shows the rotation, so option D is wrong.

Concept Tested: Scroll Housing

See: Scroll Housings

10. What is a roof curb?

  1. The flat base plate at the bottom of a rooftop fan
  2. A raised frame built into the roof around the opening, flashed into the roofing, on which the fan sits
  3. A hood mounted over the outdoor side of a wall fan to keep rain and snow out
  4. The skirt that surrounds the impeller of a downblast roof exhauster
Show Answer

The correct answer is B. A roof curb is a raised frame built into the roof around the opening and flashed into the roofing so water cannot get in. The fan's curb cap, the flat base plate at the bottom of the fan (option A), rests on top of it. The curb lifts the fan above standing water and snow. When quoting a roof fan, confirm whether the curb comes with the fan or from others, and that it fits the fan's curb cap.

Concept Tested: Roof Curb

See: Roof Curbs